Abstract: A tradeoff is often required between soil reclamation and crop productivity in sustainable agriculture. However, the critical constraints have caused soil water scarcity, nutrient impoverishment, and low hydro-thermal-nutrient use efficiency in the dryland loess regions of Northern China. This study aimed to evaluate the synergistic soil improvement via the combined application of flue gas desulfurization (FGD) gypsum and glutamic acid fermentation by-products (residue and liquid). This study aims to provide theoretical references and technical support for the green productionof maizae in dry plateau areas, the improvement of soil quality, and the efficient and green development of dryland agriculture. A systematic plot experiment was conducted in the maize field of Jinzhong dryland region, Shanxi Province, China, during an extremely arid growing season in 2024. Seven treatments included: a control (CK, without amendment), and two composite amendments at three rates (7.5, 15.0, and 22.5 t/hm²). Amendment I was formulated with 60% FGDG and 40% glutamic acid liquid, while Amendment II consisted of 60% FGD gypsum and 40% glutamic acid liquid. The parameters were then determined at the harvest stage, including key soil parameters in the 0–20 cm layer, water-soluble ion compositions, and maize agronomic traits. The results demonstrated that both composite amendments exerted significant modulating effects on soil water-salt-nutrient dynamics. In terms of hydrological properties, Amendment-II exhibited a highly stable water retention (P<0.05), with the soil water content increasing by 22.9% to 30.3% among all dosages, whereas Amendment-I shared a significant effect (25.6% increase, P<0.05) only at the highest rate (22.5 t/hm²). Furthermore, both amendments significantly mitigated alkalinity (P<0.05) in soil reaction, thus reducing pH values by 0.3 to 0.7 units. Specifically, Amendment-I treatment demonstrated a superior acidification potential (0.5–0.7 unit reduction), due to its lower raw pH (2.51) and higher residual acidity. Concurrently, soil electrical conductivity (EC) increased from a baseline of 134 μS/cm to the range of 235.0%–461.0% (Amendment-I) and 115.0%–320.0% (Amendment-II), compared with the CK. This EC enhancement was primarily attributed to the substantial input of beneficial water-soluble ions. Among them, Ca²⁺ concentrations increased by 314.3%–757.1% (Amendment-I), while SO₄^2- concentrations were 18.5 to 56.0 times those in the CK. Nutrient availability was enhanced by the synergistic mechanisms of inorganic structural improvement and organic fertility supplementation. Amendment-II shared more pronounced effects on soil organic carbon (SOC) and alkali-hydrolyzable nitrogen (AN), with the increments of 47.8%–56.0% and 17.4%–26.7%, respectively. Conversely, Amendment I enhanced 113.2%–252.0% of available phosphorus (AP) in the CK, likely due to the localized dissolution of mineral phosphates and the chelation of organic acids in the fermentation residue. Maize grain yield responded dose-dependently to amendment application. Amendment I achieved the maximum yield increment of 47.0%, thereby outperforming the 30.1% increase recorded for Amendment II. Random Forest modeling was implemented to identify soil EC, with the AN, SO₄^2-, Ca²⁺, SOC, AP, AK, and Mg²⁺ as the most influential factors for the maize yield. The EC contributed the most, with the highest relative importance (14.6%). The input of secondary-element cations (Ca²⁺ and Mg²⁺) and anions (SO₄^2-) also served a nutritional and stress-resilience role rather than inducing salinity stress under the nutrient-limited conditions of dryland degradation. Agronomically, yield improvements were attributed largely to hundred-grain weight (up to 24.5% increase) and ear diameter, indicating optimal source-sink dynamics during the grain filling stage. In conclusion, the optimal strategy was taken as the synergetic application of Amendment-I at a rate of 22.5 t/hm² for soil reclamation and yield enhancement in the dryland region. While substantial short-term benefits occurred under extreme drought conditions, multi-year fixed-site monitoring was essential to evaluate the long-term environmental safety, salt accumulation kinetics, and the potential bioaccumulation of trace heavy metals. This finding can provide a scientific basis for the high-value utilization of industrial by-products in dryland agriculture.